The invention relates to a direct injection internal combustion engine.
EP 0,560,991 B and EP 0,580,389 B disclose means for NOx reduction in internal combustion engines with lean operation. The principle applied is storage of the NOx produced, especially during lean operation of the engine, in a NOx storage catalyst and release of the stored NOx with simultaneous reduction by brief rich operation of the engine. This NOx conversion is also suitable especially in direct injection internal combustion engines. The NOx conversion allows relatively high conversion rates to be obtained where, especially for avoidance of raw NOx emissions, exhaust-gas recirculation may be used in combination with the NOx storage catalyst.
Exhaust-gas recirculation reduces raw NOx emissions. This measure is especially important in direct injection Otto engines with a NOx-reducing exhaust-gas aftertreatment system in lean operation, in particular NOx storage catalysts, since the lean conversion may break down very high raw NOx emissions, as occur particularly in homogeneous lean operation at lambda=1.1 to 1.4 or also in layered lean operation at lambda=1.6 to 4, even with the use of NOx storage catalysts, possibly as a result of inhibition of diffusion at the surface of the storage catalyst.
In addition, the recirculated exhaust gas leads to retardation in combustion, which, owing to the reduced temperature of combustion, also acts to reduce NOx and produces an improvement in fuel consumption, since the effective position of fuel conversion, which in direct injection Otto engines in layered lean operation typically is too early in the cycle, is shifted in the direction of the optimal position.
In addition, the hot recirculated exhaust gas, with suitable metering, may also lead to the stabilization of combustion in layered lean operation, since mixture formation, which must proceed on very small time scales owing to late injection in this type of operation, is supported by the increased temperature of exhaust-gas recirculation.
However, the portion of recirculated exhaust gas in the combustion chamber should not be too high, in order to provide enough fresh gas for the combustion of fuel. When exhaust-gas recirculation rates are too high, incomplete combustion takes place, owing to which fuel consumption and HC/CO emissions increase and the quietness of operation of the engine decreases.
The external exhaust-gas recirculation usually carried out, i.e., tapping of exhaust gas after the combustion chamber, in particular at the exhaust-gas manifold, and return to the intake side of the engine, permits homogeneous distribution of the exhaust gas to the individual cylinders by structural means only at great cost. In addition, the exhaust-gas recirculation rate in the dynamic operation present in an internal combustion engine, due to the time delay in the exhaust-gas recirculation line and the intake volume and the changing pressure conditions in the intake and exhaust-gas sides, is hard to adapt and adjust to desired specifications. Accordingly, the exhaust-gas recirculation rate may vary considerably among individual cylinders and a drop below undesired minimal values or exceeding undesired maximum values cannot be ruled out.
Alternatively or in addition to external exhaust-gas recirculation, internal exhaust-gas recirculation is well known, in which due to displacement of intake and exhaust times with respect to each other by displacement of intake camshafts in the direction of “EARLY”, retention of a residual gas portion in the cylinder is made possible. The advantage of this method is that, in addition to accurate distribution to individual cylinders, the residual gas already participates in the next combustion operation and the dead times described above, and the large deviations from a desired specification, largely disappear. Because of the high temperature of the internally recirculated exhaust gas, the influence on mixture formation is also more definite and can be used more selectively.
The advantages described of internal exhaust-gas recirculation are utilized in the first direct injection DI Otto engines found on the market, which in addition to external exhaust-gas recirculation also have internal exhaust-gas recirculation with intake camshaft displacement and exhaust-gas purification by means of NOx storage catalysts. For mixture formation in these internal combustion engines, a swirl concept is used for charging, in which a rotational movement is imparted to the drawn-in gases in the cylinder, the axis of rotation running approximately parallel to the piston movement/cylinder axis. At the same time, a stationary swirl is produced in the combustion chamber, into which the stream of fuel is injected and conveyed to the spark plug. In conjunction with a NOx storage catalyst, such combustion processes already have quite low NOx emissions.
The object of the present invention is to optimize, in a direct injection internal combustion engine with NOx-reducing exhaust-gas aftertreatment, the course of combustion together with exhaust-gas aftertreatment so that especially low NOx emission values are obtained.